The set of processes ensuring high fidelity replication and segregation of genomes is comprised of both structural and regulatory steps. One fundamental aspect of chromosome segregation is the requirement for association of Sister chromatids, such that each member will be oriented towards a different spindle pole in metaphase alignment. Although many proteins required for sister chromatid cohesion have been identified in the past several years, little is known about how the highly specific and very tight chromatin associations are formed. We have recently found that two DNA replication-associated proteins are required for robust sister chromatid cohesion in budding yeast, substantially augmenting the evidence for establishment of cohesion during DNA synthesis. Here we propose to pursue the contributions of one of them (S. cerevisiae CTF18p) in detail, with the goal of understanding functional relationships between proteins that act in concert with the replication fork and proteins that form the physical bridges that hold sisters together until anaphase. Our approaches include investigation of both physiological and molecular functions of Ctfl8p, using standard molecular genetic techniques. We also propose development of a novel method for visualizing alterations of mitotic chromosome structure in budding yeast. This new method has the potential for substantially refining our view of mitotic chromosome structure, as well as differentiating among abnormalities exhibited by different mutants. The proteins that mediate structural and regulatory steps governing chromosome behavior in cell division are highly conserved throughout eukaryote biological systems. The aneuploidy arising from failures in chromosome transmission has significant consequences for human health, especially in the development of somatic disorders such as cancer and polycystic kidney disease, as well as constitutional aneuploidies in which a zygote is established from gametes of abnormal chromosome number. The relationship between DNA replication and sister chromatid cohesion is a newly appreciated aspect of the chromosome cycle, and it is anticipated that studies in the yeast model system will set the stage for the development of hypotheses to be tested in human cells, and avenues for understanding when or if dysfunction of this protein contributes to human aneuploidy.